Vinyl chloride is typically produced by pyrolysis of ethylene dichloride (1,2-dichloroethane), the latter being typically produced by an oxychlorination process wherein a gaseous mixture of ethylene, hydrogen chloride and oxygen is contacted with a Deacon-type catalyst at a moderately elevated temperature. Ethylene dichloride so produced typically contains unsaturated hydrocarbon and chlorohydrocarbon impurities, chief among which are benzene and trichloroethylene.
The presence of these unsaturated impurities tend to decrease the rate of the pyrolysis reaction of ethylene dichloride to vinyl chloride with the consequence that in order to maintain a given vinyl chloride production rate, the pyrolysis furnace must be operated at a higher temperature which results in a more rapid coking rate necessitating a higher than normal frequency of plant outages in order to decoke the furnace.
Unsaturated hydrocarbons or chlorohydrocarbon impurities are not easily or economically separated from ethylene dichloride by conventional distillation techniques. For example, trichloroethylene forms an azeotrope with ethylene dichloride and benzene (boiling point, 176.2.degree. F.) has a volatility relative to ethylene dichloride (boiling point 183.degree. F.) of only about 1.10. Consequently, purification distillation of ethylene dichloride containing unsaturated hydrocarbon and chlorohydrocarbon impurities results in considerable quantities of these impurities being carried over with the ethylene dichloride due to both azeotropic entrainment in the case of trichloroethylene and to low relative volatility vis-a-vis ethylene dichloride in the case of, for example, benzene.